Rework typing notifications

Previously, the bot only had rudimentary typing notification support.

It used to send a single notification when starting a long task
and did not bother with notifications anymore.
By default matrix-rust-sdk gives these notifications a validity of 4
seconds, so it would expire shortly. If the bot takes longer to respond,
you'd see the typing notification expire and wonder if a response is
coming.

Another edge case is the bot sending an answer quicker and the typing
notice still being on. Some clients (like element-web) seem to hide the
typing notice when a new message comes, so they don't experience this as
problematic.

The reworked typing notification system should be robust:

- typing notices are sent continuously, until the bot finishes doing
  work
- if the bot is performing multiple actions in a room (even for
  different people), typing notices would continue to be sent until the
  bot becomes idle
- as soon as the bot becomes idle, a "not typing anymore" notice is sent
  to clear the state

src/controller/image/generation.rs

@@ -56,8 +56,6 @@ pub async fn handle_image(
         original_prompt.to_owned()
     };
 
-    message_context.room().typing_notice(true).await?;
-
     let span = tracing::debug_span!(
         "image_generation",
         agent_id = agent.identifier().as_string()
@@ -139,8 +137,6 @@ pub async fn handle_sticker(
         return Ok(());
     };
 
-    message_context.room().typing_notice(true).await?;
-
     let span = tracing::debug_span!(
         "sticker_generation",
         agent_id = agent.identifier().as_string()